This invention relates generally to the art of geophysical exploration using artificially induced seismic waves. It is particularly concerned with a system for generating a series of seismic wave groups such that the addition of the frequency spectra of all of the seismic wave groups gives, in effect, essentially the frequency spectrum desired for a seismic exploration in a given area.
Geophysical prospecting using artificially induced seismic disturbances has found wide application in the search for petroleum and other products. In the reflection method, it is the general practice to initiate a seismic disturbance at a point on or near the surface of the earth. Part of the seismic waves generated travel downwardly in the earth until they encounter discontinuities in the earth's structure in the form of various substrata and layering. These discontinuities have the effect of reflecting a portion of the seismic waves back toward the surface of the earth. By arranging a number of vibrational pickups known as geophones at spaced distances from the point of initiation of the artificial seismic disturbance, it is possible to detect the arrival of the reflected seismic waves at the surface of the earth. These detected waves are translated into electrical signals which are indicative of the character of the ground motion and are usually referred to collectively as a reflection signal or seismogram which is in effect a composite signal made up of a plurality of electrical waves. Skilled interpreters analyze such seismograms and are able to determine the shape and depth of subsurface layering and the likelihood of finding accumulations of such minerals as oil and gas.
The artificially induced seismic disturbance has most frequently been produced by initiating a dynamite explosion in a shothole drilled in the surface of the earth. The firing of the dynamite explosives produces an impulsive wave of high energy content over a short duration. In recent times, mechanical vibrators have been employed to initiate the artificial seismic disturbance. These vibrators impart a low amount of energy over a long period of time such that the total energy input with time can be equal to that used with an explosive source of energy.
One of these seismic exploration systems using a mechanical vibrator is, for example, the Vibroseis system, of which U.S. Pat. No. 2,688,124 to Doty et al is typical. The Vibroseis system employs a unique signal usually consisting of a variable frequency vibration applied near the surface of the earth for a substantial period of time, for example 13 seconds. The system makes use of the fact that the waves received at the various geophone locations set up to receive the vibrations reflected from subsurface discontinuities are ultimately cross-correlated with a signal representing the vibrations transmitted into the ground to produce a correlated signal in which the various reflected waves appear as discrete wavelets. An essential requirement of the Vibroseis system is that the frequency of the vibration continuously varies and no two cycles of the transmitted wave should be of essentially the same frequency.
Another approach employing a mechanical vibrator is found in U.S. Pat. No. 3,182,743 to McCollum in which one after another, a series of wave trains of essentially sinusoidal wave form are transmitted into the ground. Each wave train is a discrete event of substantial by constant frequency, but the center frequency of each wave train in the series is different and are related by values of an arithmetic progression. The received waves due to each wave train are combined together in such a manner such that one of the central half cycles is in phase. The resultant waves after combination appear essentially like those obtained from the last step of the Vibroseis system, or like those of the systems employing a dynamite explosion.
U.S. Pat. No. 3,259,878 to Mifsud describes a method of seismic exploration quite similar to that described in the McCollum patent.
U.S. Pat. No. 3,221,297 to Smith et al also describes a method of seismic exploration in which discrete constant frequencies are transmitted into the earth and the ratio of the amplitude in phase of the transmitted waves and the received waves is recorded.
U.S. Pat. No. 3,568,142 to Landrum describes still another method very similar to the methods described in the Mifsud and McCollum patents except that provision is made for multiple filters and delay units so that the succession of wave trains transmitted in the earth can follow one upon the other without waiting for the vibration of one to be subsided before beginning the transmission of the next.
There are a number of advantages obtained by use of discrete wave groups of constant frequency in a series as taught by McCollum, Mifsud, and Smith et al. In these methods, each transmission of seismic waves is discrete and separate so that there is sufficient time for vibrational waves to subside and terminate the recording of waves due to one transmission before the beginning of the next transmission. Thus, there is no overlap on the resulting record of vibrations returning from deep subsurface layers due to a previous transmission of seismic waves at the same time that seismic waves are being returned from shallow subsurface layers due to a subsequent transmission. Another significant advantage is that since each wave group has primarily a single frequency, electrical wave filtering may be employed to pass only that frequency and suppress any frequencies due to extraneous noise. In addition, arrays of geophones and vibrators may be employed to act as filters for a particular frequency. One final and very significant advantage is that the method provides a simple and effective means of introducing greater energy at higher frequency and provide greater resolution for stratigraphic trap exploration. It is well known that the higher frequencies of seismic waves travelling through the earth are attenuated to a greater extent than are the lower frequencies. It is these higher frequencies that are required to provide the resolution for fine subsurface detail such as stratigraphic traps which are quite favorable to the accumulation of petroleum. For these higher frequencies multiple inputs of the same wave groups may be applied to the earth and summed in the output to create a higher energy level for resolution of the stratigraphic traps and other fine subsurface detail.
One of the problems associated with the technique of applying multiple wave groups of constant but different frequencies is that a large number of wave groups was thought to be required. In the use of a surveying system employing an explosive the duration of the explosive is just a fraction of a second and the recording time following detonation of the explosive is generally from four to seven seconds depending upon the depth in the earth that is of interest. Following each detonation of an explosive the survey may proceed to a new position. In a surveying system in which multiple wave groups of constant frequency are applied, it is usually most practical to apply each wave group sequentially and continue recording until all vibrations resulting from that input have subsided before beginning the application of the next wave group. The duration of each wave group may be only a fraction of a second but after each transmission there must be waited an elapsed time of four to seven seconds for recording of reflected waves from deep horizons. The more separate wave groups required to cover a given frequency spectrum the longer will be the time required to complete the survey for a given position. Thus, the number of wave groups required may be a critical factor in determining whether this method is feasible and economical. The longer equipment and personnel are required in the field the more expensive the survey. No matter how effective are the results, the economics of the situation may prohibit the practical application of the method.